array_flip entity.inc:173 on user_load();

Specially when I am running cron, indexing content or removing content, I get these warnings:

" array_flip(): Can only flip STRING and INTEGER values! entity.inc:173 "

Possibly the source of the issue: apachesolr.api.php: $user = user_load(array('uid' => $entity->uid));

Checking user_load function on api.drupal.org for the user_load function, you can see that it is not looking for an array, but an integer.

I ran into this information on stack overflow but despite I greped the code for this and found it I may have misunderstood the source of the problem.